Recording neural activities plays an important role in numerous applications ranging from brain mapping to implementation of brain-machine interfaces (BMI) to recover lost functions or to understand the mechanisms behind the neurological disorders such as essential tremor, Parkinson's disease and epilepsy. It also constitutes the first step of a closed-loop therapy system which additionally employs a stimulator and a decision mechanism. Such systems are envisaged to record neural anomalies and then stimulate corresponding tissues to cease such activities. Methods for recording the neural signals have evolved to its current state since decades, and the evolution still goes on.
Current clinical practice in recording electrical activities of the brain is dominated by electroencephalography (EEG) which is a non-invasive procedure performed along the scalp. Another type of EEG, intracranial EEG (iEEG; also known as electrocorticography (ECoG)) is an invasive procedure which is performed by placing an electrode matrix (or array) onto the cortex following craniotomy. Intracranial EEG is, for instance, employed for epileptic focus localization prior to a respective surgery which is performed to treat certain types of epilepsy.
Currently, iEEG is performed by using electrode arrays composed of millimeter-sized passive electrodes. See Carrette E. et al., Clinical Neurology and Neurosurgery, 112(2):118-126, 2010. Neural signals acquired via these electrodes are transferred to an external recording device via transcutaneous wires. However, the wired connection through the skull during the monitoring period increases the risk of the cerebrospinal fluid (CSF) leakage, or worse, infection of the CSF. So far, all the implantation procedures are achieved using transcutaneous wired devices, associated with potential serious complications in up to 25% of cases, such as intracranial infections or CSF leakage. See Hamer H. M. et al., Neurology, 58(1):97-103, January 2002. Finally, patients are permanently connected to a recording station through multiple connecting cables, leading to decreased comfort and autonomy. This situation reduces the patient mobility and affects psychological state of the patient and more importantly limits the monitoring period. Sealing the craniotomy area up is anticipated to eliminate or at least minimize these risks.
The demand from neural systems increases continuously in terms of quality and quantity of extracted information with the improvements in the microsystems and microelectronics. This demand drives the technology from external recording systems to in-vivo recording systems. As new technological developments take place, so does the opportunity to improve current designs or performance, decrease power requirements or cost, and/or minimize complications associated with chronic implantation. Implanted neural recording systems are expected to offer better spatial and temporal resolution, thanks to implantable microelectrodes and on-site processing microelectronics.
Implantable biomedical devices usually require an isolated volume from the surrounding tissues to prevent the cross-interaction between the device and the tissues. This isolation, however, should not deteriorate the performance of the device. Independent of the location of the implantation, the isolated volume is to be provided by the human body volume. Therefore, a subtractive process is to be performed at the location where the implant is placed. Specifically, for the brain implants, the volume is usually obtained by forming a recess on/inside the cranium. It should be noted that, generally, it is required to implant the biomedical device as close as possible to the point-of-interest. See U.S. Pat. Pub. No. 2005/0113744. As another note, the dimensions of the recess which will contain the implantable device is limited from the upper bound by the physiological limits of the human anatomy and from the lower bound by the content of the device, for instance, electrical and mechanical parts.
U.S. Pat. Nos. 7,212,864 and 9,084,901 proposed a method to implant a biomedical device into the head of a patient. They suggest placing some modules that are completely covered by an overmold between the cranium and the scalp, while the rest of the modules that are partially covered with another overmold into a recess in the cranium. The purpose is to distribute the parts of the implant in such a way that most of the implant volume is to be placed into a recess in the cranium, and therefore, less implant volume is required between the cranium and the scalp. However, these approaches are uncomfortable in case the module placed between the cranium and the scalp should be replaced or removed, and could be aesthetically unpleasant.
A further recent approach relates to drilling a Burr hole and implant the biomedical device therein. Silay K M et al., Sensors Journal, IEEE, vol. 11, no. 11, pp. 2825-2833; Wireless Power Transfer and Data Communication for Intracranial Neural Implants Case Study: Epilepsy Monitoring, Ph. D. thesis by Gürkan Yilmaz. By this way, the performance of the system has been improved without sacrificing from the surface area of the Burr hole. This approach however provides little space for placing a plurality of active devices for monitoring or influencing brain activity, for adapting several (bidirectional) data communication components or (wirelessly-driven) power supply and, importantly, for managing the thermal burden generated by the active elements of an implant. See Patrick D. Wolf, Thermal considerations for the design of an implanted cortical brain-machine interface (BMI). In William M Reichert, editor, Indwelling neural implants: Strategies for contending with the in vivo environment. CRC Press, Boca Raton (Fla.), 2008).
Some additional patent applications and publications, including U.S. Pat. Nos. 7,346,391, 8,165,684, and U.S. Pat. Pub. Nos. 2001/0051819, 2004/0176817 and 2007/0255338, provide different scenarios concerning cerebral implantable medical devices or brain-machine interfaces. Especially in this latter case, none of those documents addressed or focused on some of the practical problems arising during the operation of said devices, namely the need of an as much as possible minimally invasive implant for avoiding discomfort of the patient, the optimization of the power supply of the implanted device and/or of the bidirectional data communication with external systems so to avoid external structures such as wired cables or catheters, and at the same time the need of dissipating the heat load generated during operation of an active implant. Accordingly, despite all the advancements in the field of skull replacement and brain interfaces, additional and more advanced solutions are still necessary and desired.